1. Field of the Invention
This invention pertains in general to a method for forming raised source and drain regions in a semiconductor transistor device and, more particularly, to a method of forming self-aligned raised source and drain regions in a metal-oxide semiconductor ("MOS") transistor device.
2. Description of the Related Art
As transistor dimensions in integrated circuits become smaller, the formation of ultra-shallow transistor junctions has become an important consideration in device formation. A mechanism that allows the formation of ultra-shallow junctions is through the formation of raised, or elevated, source and drain regions.
The material that constitutes the raised source and drain regions is generally the same as the device substrate, which in most instances is silicon. The raised source and drain regions may also be used as alternative self-aligned contacts in ultra-large silicon integration ("ULSI") MOS devices, but they first need to be metalized. A metalized silicon, or silicide, is silicon combined with a metal, and is formed over the source and drain regions. Silicides may be provided as a Group VIII silicide (PtSi, Pd.sub.2 Si, CoSi.sub.2 and NiSi.sub.2) or as TiSi.sub.2.
In a standard CMOS process, a low-density implant step is performed by using a polysilicon gate as a mask to form what will later become lightly-doped drain ("L.sub.DD ") regions in the device substrate. An oxide layer is then formed over the device substrate and surrounds the gate. The oxide layer is etched, leaving oxide layers contiguous with the sides of the gate. These oxide layers are known as oxide spacers. Because the lateral dimensions of the spacers are usually very small, subsequent formation of suicides may bridge the separation between source and drain silicides and cause the gate to become short-circuited. This is known as salicide bridging.
The conventional method continues by forming raised source and drain regions. This may be accomplished through either epitaxial growth of silicon or deposition of silicon. Epitaxial growth requires high temperature that may cause excessive diffusion of impurities implanted to form the L.sub.DD regions. The step of depositing silicon to form raised source and drain regions may be accomplished through sputtering of silicon. A conventional sputtering process bombards a silicon target, or a sheet of pure silicon, with energetic ions to dislodge a plurality of silicon atoms from the target. Because the dislodged silicon atoms move in arbitrary directions, the silicon atoms will coat the entire surfaces of the spacers with silicon, which would require additional steps to remove the silicon from the undesired surfaces of the spacers. However, the silicon removal steps will damage the spacers, which would require additional steps to reform the spacers.